There is a need for an optical fiber, having a halide core with a halide cladding, for the transmission of infrared electromagnetic radiation, which is capable of economical mass production, yet minimizes radiation transmission losses. Transmission losses arise from impurities in the halide starting materials, from impurities in the halide materials introduced from the extruder and the extruder die, from irregular surface boundary conditions at the core-cladding interface, from non-uniform mixing of the core and the cladding materials at the interface, and from the presence of grain boundaries between the numerous crystals forming the core. Each of these problems decreases the transmission efficiency of the core material by increasing the scatter of the transmitted electromagnetic radiation.
U.S. Pat. No. 4,253,731 discloses a metal halide (AgBr) fiber (or core) which is clad with another metal halide (AgCl). The silver bromide core has a fine-grained crystalline structure which is produced by an extrusion process, preferably in the low temperature range set forth in FIG. 3 of the patent. In the extrusion process, a coaxial or composite billet, i.e. a sleeve of cladding surrounding a cylindrical core, is heated in a single chamber and extruded through a diamond die according to FIG. 2 of Pat. No. 4,253,731.
Chen, D. et al, "Fabrication of Silver Halide Fiber by Extrusion," Fiber Optics: Advances in Research and Development, ed. by B. Bendow et al, Plenum, N.Y., p. 119-122, 1977, also, discloses a clad fiber similar to the one described in U.S. Pat. No. 4,253,731
Japanese Patent Application No. 1980-87508, published in the Japan Patent Journal No. 1982-13410, discloses two embodiments of an infrared optical fiber and methods of producing each. In the first embodiment, a cladded optical fiber is produced by a first extrusion step from a composite billet. The resulting cladded fiber is then subjected to a second extrusion step during which a plastic coating, e.g. polyethylene, polypropylene, nylon-6, polyacetal or acrylic, is applied on the cladded fiber.
According to the second embodiment of Japanese Patent Application 1980-87508, an uncladded optical fiber having a plastic coating is produced in a two-step extrusion process. An uncladded metal halide billet, i.e. not a composite billet, is extruded into a fiber during a first step, and a plastic coating is then extruded around the fiber in a second step.
U.S. Pat. No. 4,678,274, assigned to the assignee of the hereof patent application, discloses a cladded optical fiber, having a halide core and halide cladding extruded from a composite billet having a covering of a polymer film.
Pinnow, D. A. et al, "Polycrystalline Fiber Optic Waveguides For Infrared Transmission", Applied Physics Letters, Vol. 33, No. 1, Jul. 1, 1987, disclose optical fibers formed by extrusion of thallium bromide or thallium bromoiodide. The extruded fibers are then sheathed in a loose-fitting polymer sleeve. The core fibers are polycrystalline.
Japanese Patent Application No. 1981-140929, published in Japan Patent Journal No. 1983-43404, discloses an improved extrusion die for producing optical fibers from halide billets. Contamination of the billet from chamber wall impurities is reportedly avoided by a die which has a diameter less than the diameter of the billet. The die has a beveled attack surface which meets the billet. The die, in effect, shears off an annular portion of the billet which contains the contaminants.
Vasil'ev, A. V. et al, "Single-Crystal Fiber Waveguides For The Middle Infrared Range," Sov. J. Quantum Electron., Vol. 11, No. 6, Jun., 1981, disclose a single-crystal halide optical fiber which is grown in capillaries. The single-crystal fibers do not have a cladding.
U.S. Pat. No. 4,583,821 discloses a cladded optical fiber, formed from mixed AgBr/AgCl crystals surrounded by a protective layer. The clad fiber is extruded from a composite billet as discussed in U.S. Pat. No. 4,253,731. An extrusion temperature between 100.degree. C. and 380.degree. C. is reported.
U.S. Pat. No. 4,552,434 discloses an optical fiber having a halide core and a halide cladding (See FIG. 9a) which is produced by placing a core billet into a sleeve of cladding and drawing the sleeve into contact with the core billet. A gap of about 0.01-0.1 mm is maintained between the billet core and the sleeve cladding before the drawing step. The following drawing temperatures are disclosed for the halide material of Pat. No. 4,552,434: 120.degree. C.-358.degree. C. for KRS-5; 100.degree.-370.degree. C. for silver chloride; 180.degree.-370.degree. C. for cesium iodide (melting point 626.degree. C.).
Mimura, Y. et al, "CsBr Crystalline Fiber For Visible and Infrared Transmission," Japanese Journal of Applied Physics, Vol. 20, No. 1, p. L17-L18 (January 1981), disclose a cesium bromide optical fiber which is inserted into a Teflon.RTM. jacket.
Japanese Public Patent Disclosure Bulletin No. 56-104302 discloses an optical fiber with a halide core and a halide cladding. This fiber is produced by forming a composite billet, placing the billet into a sealed metal pipe, and drawing down the pipe's diameter, i.e. cold working, until the desired fiber diameter is reached.
Harrington, J., "Crystalline Infrared Fibers," Proc. Soc. Photo-Opt. Inst. Eng., Vol. 226, February 1981, and Harrington, J. et al., "Scattering Losses in Single and Polycrystalline Materials for IR Fiber Applications," Adv. in Ceramics, vol. 2, pp. 94-103 (1981) studied the scattering losses in single crystal and polycrystalline KCl and KRS-5. The authors report that polycrystalline materials scatter more radiation than single crystal materials.
Sakuragi, S. et al, "IR Transmission Capabilities of Thallium Halide and Silver Halide Optical Fibers," Adv. In Ceramics, Vol. 2, pp. 84-93, 1981, describe experiments with unclad halide fibers.